Furnace for heat treating objects under pressure

ABSTRACT

In manufacturing sintered bodies, a compacted charge of powder forming a billet is enclosed in a sealed casing of soft metal such as steel from which the air has been evacuated. The casing and powder are subjected to pressure at high temperature in a furnace chamber to sinter the powder. Before its introduction into the chamber, the charge is heated outside the furnace to treating temperature. The charge is inserted from below into the furnace, after which the bottom opening of the furnace is closed and pressure medium is introduced. The furnace has an insulating sheath within the furnace chamber depending from the top of the chamber and terminating near its bottom. The material of the casing is so chosen that there will be substantially no interchange of material especially of carbon, with the material of the compacted billet.

Umted States Patent [151 3,6 Lundstrom [4 Oct. 3, 1972 [54] FURNACE FORHEAT TREATING [56] References Cited RESSURE OBJECTS UNDER P UNITEDSTATES PATENTS [721 Inventor: 3,628,779 12/1971 Lundstrom ..266/5 E ween Primary ExaminerGerald A. Dost [73] Assignee: Allmanna SvenskaElektrlska Ak- A" Jennings Bailey, Jr.

tiebolaget, Vasteras, Sweden [22 Filed: Sept. 1, 1970 ABSTRACT Inmanufacturing sintered bodies, a compacted charge [21] Appl' of powderforming a billet is enclosed in a sealed cas- [30] Foreign n m priorityData ing of soft metal such as steel from which the air has beenevacuated. The casing and powder are subjected Oct. 24. I968 Sweden l436/63 to pressure at high temperature in a furnace chamber to sinter thepowder. Before its introduction into the chamber, the charge is heatedoutside the furnace to treating temperature. The charge is inserted fromRelated Apphcauon Data below into the furnace, after which the bottomopen- [63] Continuation-impart o Set. Oct. ing of the furnace is closedand pressure medium is in- 16, 1969, abandoned. troduced. The furnacehas an insulating sheath within the furnace chamber depending from thetop of the chamber and terminating near its bottom. The material of thecasing is so chosen that there will be substan [52] US. Cl. ..266/2 anyno interchange of material especially of carbon gf 6/2 R I R with thematerial of the compacted billet.

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saw 3 OF 4 INVENTOR. HANS LUNDSTR'OM FURNACE FOR HEAT TREATING OBJECTSUNDER PRESSURE RELATED APPLICATIONS Thisapplication is acontinuation-inpart of application Ser. No. 866,885, filed Oct. 16, 1969and now abandoned.

BACKGROUND OF THE INVENTION 1 Field of the Invention The presentinvention relates to a method, when pressure-sintering powder bodiesusing a vertical furnace provided with pressure chamber, forsimultaneously treating a material at high temperatures, up to l,500 C.,and high pressure, preferably 500 bar and above.

2. The Prior Art A furnace for treating objects under high pressure isearlier disclosed in the US. Pat. application, Ser. No. 855,911 filed onSept. 8, 1969 and entitled Furnace for Heat-Treating Objects Under HighPressure, assigned to the assignee of the present application, now US.Pat. No. 3,598,378, issued Aug. 10, 1971.

If metal powder is enclosed in a sheath, the sheath evacuated, sealedand, together with its contents, subjected to high pressure and hightemperature simultaneously, the metal powder sinters to a solid body ata lower temperature than at atmospheric pressure.

By sintering under high pressure it is possible to achieve greaterdensity than with sintering at atmospheric pressure and thus betterproperties of the finished product in many respects. The method seems tohave great advantages in the production of alloys which, withconventional casting of molten metal into billets, easily becomecoarse-grained and have a tendency to segregate upon solidifying. Bymanufacturing these alloys of fine-grained powder and then sinteringbodies formed from the powder with hot isostatic compression, ahomogenous and very finegrained structure is obtained.

SUMMARY OF THE INVENTION Hitherto the method has only been suitable forlaboratory use or for production on a very limited scale. However, themethod according to the invention is suitable for industrial productionso that pressuresintering can be a realistic alternative in many fieldsto other methods of manufacture. The method is essentially characterizedin that the evacuated casing filled with powder is heated at atmosphericpressure outside the furnace provided with a pressure chamber, insertedfrom below in the furnace chamber which is heated to sinteringtemperature and located inside the pressure chamber, through a chargingopening, and the pressure chamber closed, a pressure medium such ashelium or argon is pumped into the pressure chamber, pressure andtemperature are set at suitable values for sintering for a predeterminedlength of time, the pressure is reduced, the pressure chamber opened andthe charge taken out through the charging opening in the bottom of thefurnace. For certain purposes the pressure medium may be nitrogen orhydrogen. The powder in the evacuated sheath may be isostaticallycompacted before heating at atmospheric pressure before insertion intothe pressure sintering furnace.

The invention also relates to a furnace for carrying out the method.

Furnace for pressure sintering normally comprise a cylindrical pressurechamber suspended in a stand and consisting of a high pressure cylinderwith end closures projecting into it, a press stand which surrounds thehigh pressure chamber during the compressing operation and takes up theaxial forces which a pressure medium in the pressure chamber, usually aninert gas, exercises on the end closures, insulation arranged in thepressure chamber between a furnace chamber and the walls of the pressurechamber, the insulation consisting of a cylindrical sheath withinsulating lid and bottom, and transport members to move the press standbetween a charging position and a compressing position. Furnaces havinghigh pressure cylinders with threads at their ends and threaded endclosures which take up the axial forces are practical at reasonablepressures, but are very risky at high pressures. Argon is usually usedas pressure medium. At high pressure argon has very high density but atthe same time low viscosity, only 4-5 times that of air at atmosphericpressure, and is thus very mobile. Since, with respect to the density,it also has great heat capacity, it is important that the insulationsurrounding the furnace chamber itself is designed in such a way thatthe least possible convection is obtained between the furnace chamberand the inner walls of the pressure chamber so that the heat losses arenot unreasonably great. Conventional furnaces have apermanent lower endclosure provided with insulation, as the bottom. This supports aninsulating cylindrical sheath and heating elements. The furnaces arecharged from the top. It is therefore necessary to have a detachable lidfor the sheath, which involves considerable sealing problems. These aresuperable if the furnace can be sealed and opened cold. If the sealleaks gaps may arise through which hot gas may seep from the furnacechamber. The lid seal cannot be cooled directly and it is hardlypossible to obtain a reliable seal if the lid is placed on a hotfurnace. So that the internal parts of the furnace shall not besubjected to temperature shocks when the lid is opened and the hot gasesarise and are replaced by cold air, the furnace must cool before it isopened. Rapid cooling of the inner metal of the sheath may also damagethe insulating layer inside the metal so that it cracks and theinsulating capacity decreases. Furthermore, it is not advisable for thehot sheath or heating elements to be subjected to oxidizing atmosphere.If there is molybdenum in the sheath of heating elements they aredestroyed in a very short time. If air penetrates into the furnacechamber it must be evacuated and rinsed to remove air particles, andthis takes time. All this contributes to a lengthy cycle time which, inview of the high cost of the furnace, is a great disadvantage.

The furnace according to the invention is designed so that it can bekept continuously filled with inert gas and thus also continuouslyheated to operating temperature since the insulating sheath and heatingelements are not subjected to rapid alterations in temperature orcontact with oxygen in the air. It can thus be emptied without previouscooling and charged with heated new powder bodies, especially billets ofhigh-speed steel for rolling mills. It takes an extremely long time toheat billets to an even temperature all the way through, especiallylarge billets. With the process suggested the billets can be heated tosintering temperature at least substantially at atmospheric pressure infurnaces of conventional type.

The time for an operating cycle can thus be kept low and the costs willbe considerably lower than with conventional furnaces.Pressure-sintering powder will thus become a method of manufacturingwhich can be generally used, for example for manufacturing ingots frompowder. The method is particularly advantageous for such alloys wherefine-grained structure, uniform alloying or bubble-free goods cannot beachieved by conventional casting methods. The furnace is characterizedsubstantially in that the upper end closure of the pressure chamber ispermanently arranged in the high pressure cylinder, that the insulatingsheath and the insulating lid are suspended in the upper end closure andthat the lower closure supports the insulating bottom and the charge andcan be raised and lowered by means of a lifting mechanism while thefurnace is being charged from below. This design prevents hot furnacegas flowing to the gap' between the sheath and the walls of the pressurechamber if there are leaks between the insulating sheath and theinsulating lid. The seal between the insulating lid and the sheaththerefore is not of the same vital importance as in earlierconstruction. It is, furthermore, permanent and is not opened when thefurnace is charged. It is also cooled effectively, which considerablyreduces the likelihood of leakage. The insulating lid is designed orinserted in the sheath so that a closed space is formed between the lidand the end closure, this space communicating with the furnace chamberonly through a pressure-equalizing opening. This space can be used forconnecting heating elements to lead-in cables and facilitates assemblyand dismantling of the furnace.

In one embodiment of heat body a tube surrounding the furnace chamber issuspended in the insulating lid. The heating elements of the furnace aresuspended on the outside of the tube and can be held in position withthe help of vertical bars having slots through which the elements pass.The tube may even be provided with internal, vertical protective bars.If the tube finishes a little way below the insulating lid and if it issuspended in this by means of said bars, openings are obtained throughwhich the elements can be drawn to the bushings in the lid. Electriccurrent and pressure medium are of course supplied through thestationary upper end closure. It is convenient to make the lower endclosure in two parts with an annular part permanently suspended in thehigh pressure cylinder and a lid arranged inside this cylinder, whichcan be raised and lowered and which supports the insulating bottom andthe charge. An insulating ring is attached to the annular part andprojects up into the insulating sheath. When heating elements aresuspended on a tube, the ring is pressed up into a gap between thesheath and the tube so that a labyrinth seal is obtained in the lowerpart of the furnace between the gap between the high pressure cylinderand the sheath and the furnace chamber itself. In this embodiment it ispossible to effectively protect the insulating sheath from contact withthe air when the lid of the end closure has been removed. Bycontinuously supplying gas to the gap between the high pressure cylinderand the insulating sheath, a downwardly flowing gas stream is obtainedwhich in the lower part is deflected and forced into the gap between theinsulating sheath and the tube carrying the heating elements. Thecontinuous supply of gas, for example argon, to this gap ensures thatthe insulating sheath and the heating elements are always situated in acontrolled inert atmosphere which, even at high temperatures, does notdamage the material. Any air can be limited to the lower part of thefurnace chamber. If the tube surrounding the furnace chamber is of amaterial, for example inconel, which is resistant to oxidization even athigh temperatures, this small quantity of air need not cause anymaterial damage. The invention thus permits great freedom of choice asto the material for most of the vital components of the furnace.Radiation protection means may also be built into the annular part ofthe end closure and are automatically lowered when the bottom is loweredso that the sealing surface between the bottom andthe annular part isprotected during charging against heat radiation from the charge. Theannular part may also form a protective or guiding ring which preventsthe charge from being inserted in such a way that the inner parts of thefurnace come into contact with the charge and are damaged.

A further matter of importance when selecting the material for thecasing is to prevent the migration or interchange of material betweenthe casing and the enclosed billet of a powder mixture to be sintered.The migration of carbon must be particularly observed.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be furtherdescribed with reference to the accompanying drawing:

FIG. 1 shows a side view of a furnace, partly in section,

FIGS. 2 and 3 sections of a part of the lower and upper parts of thehigh pressure chamber, respectively, and

FIG. 4 a horizontal section through the high pressure chamber.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the drawings, 1 designates apress stand which is movable between the position shown in the drawingand a position where it surrounds the high pressure chamber 2. The pressstand is of the type consisting of yokes 3 and 4, spacers 5 and a stripmantle 6. The stand is supported by wheels 7 running on rails 8. Thehigh pressure chamber 2 is supported by a pillar 9. It contains a highpressure cylinder consisting of an inner tube 10 and a surrounding stripmantle 11 with end rings 12 holding the strip mantle together axiallyand forming grips for brackets by which the high pressure chamber isattached to the pillar 9. The chamber 2 has an upper end closure 13which projects into the tube 10 of the high pressure chamber. Betweenthe tube and the end closure is a sealing ring 14. The furnace chamber15, in which the charge 16 is inserted, is surrounded by an insulatingsheath 17 consisting of three concentric metal tubes 18, 19 and 20,layers 21 and 22 of insulating material and two end rings 23 and 24. Thesheath 17 is suspended by means of the ring 23 in the upper end closure13 and connected to this lid closure in a gas-tight manner. In the upperpart of the sheath 17 is an insulating lid 25 consisting of a metalcasing 26 filled with insulating material 27 and provided with bushings28 for electric conductors to heating elements 47. The casing 26 isattached to an attachment ring 29 clamped between the ring 24 and theend closure 13. In the ring 29 is a groove for the sealing rings 30 and31. Between the lid 25 and the upper end closure 13 a closed space 32 isformed which communicates with the furnace chamber only through apressureequalizing opening 33. In the upper end closure is a lead-inchannel 34 with a sealing member 36 for a feeding conductor 35. In theclosure is also a channel 37 for the supply of pressure medium and agroove for the sealing ring 14. The channel 37 opens into the gap 39between the inner wall of the tube 10 and the outer tube 18 in thesheath 17. The heating elements 47 are supported by a tube 40 whichsurrounds the furnace chamber 15. The elements are attached to the outerside of the tube 40 by means of rails 41 having slots through which theheating elements can pass. The tube 40 is suspended in the lid 25 bymeans of the protective rails 43 on the inside of the tube. Between thelid 25 and the tube 40 is a gap 44, so that openings are formed throughwhich the connecting part of the heating elements can pass from the gap46 between the tube of the sheath l7 and the tube 40. Above the upperend closure 13 is a pressure plate 45 with slots for the electricconductor 35.

The pressure chamber has a lower end closure consisting of an outerannular part 50 permanently attached to. the high pressure cylinder anda lid 51 projecting into this part. In both parts of the end closure areslots for sealing rings 52 and 53. A metal ring 54 is attached to thelid 51. In this is a thick layer of insulating material 55 and a plate65 on which the charge is placed. This plate is preferably made of amaterial having good heat conductivity and may be provided with radialchannels 66 and 67 between the periphery and a central space 68. If theplate 65 has good heat conductivity the central parts of the billet 16can be supplied with heat through the plate. When the gas in the spaceis cooled it circulates from the gap 69 in through the channel 66 andout through the channel 67, which contributes to more rapid heating ofthe central parts of the billet. On the ring 50 is an insulating ring 56which projects into the gap 46 so that between the lower part of thefurnace chamber 15 and the gap 39 a labyrinth seal is formed. In thering-shaped part 50 metal rings 57 and 58 are arranged which fall alittle way away from each other during charging when the lid 51 islowered so that the sealing surface in the ring 50 against which theseal 53 abuts is protected from heat radiation from the charge 16 duringcharging. The lid 51 is attached by brackets 59 to a casing 60 whichruns along a guide 61 attached to the pillar 9. The casing can belowered by an operating cylinder 62, the operating rod 63 of which isattached at bracket 64 on the casing 60.

The casing enclosing the powder is subjected to high temperature duringsintering and a material interchange may take place between the powderand the material of the casing. For alloying substances having highdiffusion ability such as, for example, carbon, a considerablyconversion may take place. It is therefore important that the materialchosen for the casing should have approximately the same carbon activityat the sintering temperature as the material in the powder enclosed inthe casing. It has been found that the carbon activity for a casingmaterial of steel plate having 0.10% C, 0.20% Si and 0.35% Mn and powdermaterial containing 0.85% C, 4.0% Si, 6% W, 5% Mo, 2% Va and theremainder Fe is approximately the same. When the sintering takes placeat l,l50 C. at a pressure of l kbar, the alterations in the boundarylayer for the powder material mentioned are negligible.

In lieu of soft low-carbon steel, other materials such as soft stainlesssteel and other metals may be used, which are sufficiently deformable topermit the exertion of most of the pressure to the enclosed powderbillet and which do not interchange elements, especially carbon, to asubstantial degree with the material of the billet.

The device is utilized in the following manner: A charge in the form ofa cylindrical billet 16, consisting of a powder enclosed in a casingfrom which air has been evacuated, is heated to about the sintering temperature in a preheating furnace, not shown. This heated billet isplaced upon the metal plate 65, carried by the lower lid 51 in loweredposition. The lid 51 is then lifted by the cylinder 62 to the positionshown in the drawings. The press stand 1 :is then moved to the rightfrom the position of figure l to a position where it surrounds thepressure chamber 2. Pressure medium from a pressure medium source (notshown) preferably heated, is then supplied to the pressure chamberthrough the channel 37 in the upper end-closure 13. The upwardly anddownwardly directed forces upon the upper end and lower-end closures,produced by the pressure medium, are taken up by the press stand 1.During the pressure sintering of the billet 16, the temperature isadjusted and kept at a level suitable for the treatment by the heatingelement 47, connected to a current source by a feeding conductor 35.After the treatment, and after lowering the pressure in the pressurechamber, the press-stand l is moved back to the position shown in FIG.1, whereafter the lid 51 with billet 16 is lowered and the treatedbillet replaced by another preheated billet to be sintered.

When the furnace is loaded from below, a bubble of inert gas will remaininside the pressure chamber and prevent air from ruining hot furnaceparts, such as the insulating sheath or electric heating element if thefurnace is opened when still hot. It will therefore be unnecessary tocool the furnace between chargings. The powder body, for instance, inthe form of a billet, can be heated to the treating temperature outsidethe high pressure furnace. The heating time for a large diameter billetis 5-10 hours and the necessary treating time in the high pressurefurnace perhaps about 10 percent of this time. The cost of a preheatingfurnace furthermore is only a fraction of the cost of the high pressurefurnace.

The invention is of course not limited to the embodiment shown, Manyvariations are feasible within the scope of the following claims.

Iclaim:

l. Furnace for use in a method of manufacturing sintered bodies whichcomprises enclosing a charge of powder in a casing, evacuating air fromthe casing, sealing the casing, inserting the casing and its contentsinto a furnace having a furnace chamber which is enclosed in a pressurechamber, subjecting the sealed casing to high pressure at hightemperature so that the powder sinters, the evacuated casing beingheated outside the furnace, inserted from below into the furnace chamberwhich is heated to sintering temperature and located inside the pressurechamber through a charging opening, the pressure chamber being closedand a pressure medium being pumped into the pressure chamber whilesetting the pressure and temperature for sintering, for a predeterminedlength of time, which furnace comprises a cylindrical pressure chamberincluding a high pressure cylinder with a furnace chamber therein, upperand lower end closures projecting into said cylinder, insulation in thepressure chamber between said furnace chamber and the walls of thepressure chamber, which insulation comprises an insulating sheath withinsulating lid and bottom, and in which the upper end closure of thepressure chamber is permanently secured in the high pressure cylinder,the insulating sheath and the insulating lid are suspended in the upperend closure and the lower end closure supports the insulating bottom andthe charge and can be raised and lowered while the furnace is beingcharged from below.

2. Furnace according to claim 1, in which resistance elements to heatthe furnace are suspended in the upper end closure.

3. Furnace according to claim 1 in which the insulating sheath isconnected in a gas-tight manner to the upper end closure.

4. Furnace according to claim 3, in which there is a space between theinsulating lid and the upper end closure and the furnace chamber througha pressureequalizing opening connecting said space to the furnacechamber.

5. Furnace according to claim 1, comprising a tube suspended in theinsulating lid and surrounding the furpace and resistance elements toheat the furnace supported by said tube.

6. Furnace according to claim 5, in which the resistance elements arearranged on the outside of the supporting tube.

7. Furnace according to claim 6, comprising vertical rails withhorizontal slots facing the tube, the resistance elements beingsupported in said rails and passing through said slots.

8. Furnace according to claim 7, in which the vertical rails are carriedby the tube carrying the resistance elements.

9. Furnace according to claim 6, in which the tube carrying theresistance elements is suspended in the insulating lid by means of saidrails and at a distance from the lid so that openings are formed betweenlid and tube between the rails.

10. Furnace according to claim 5, in which there are gas-tight bushingsin the insulating lid for the resistance elements.

11. Furnace according to claim 1, including supply conduits for saidresistance elements passing through the upper end closure.

12. Furnace according to claim 1, in which the lower end closurecomprises a ring-shaped part permanently suspended in the high pressurecylinder and a lid inside this ring-shaped part which can be raised andlowered and which supports the insulating bottom and the char e.

13 Furnace according to claim 12, in which the ringshaped part of theend closure supports an insulating ring which projects into theinsulating sheath so that between this and the ring a narrow gap isformed.

14. Furnace according to claim 12, in which in the ring-shaped part ofthe end closure is arranged a ringshaped axially lowerable radiationprotection which, when the lid is lowered, is lowered and protects thesealing surface in the ring-shaped part from radiation from the charge.

1. Furnace for use in a method of manufacturing sintered bodies whichcomprises enclosing a charge of powder in a casing, evacuating air fromthe casing, sealing the casing, inserting the casing and its contentsinto a furnace having a furnace chamber which is enclosed in a pressurechamber, subjecting the sealed casing to high pressure at hightemperature so that the powder sinters, the evacuated casing beingheated outside the furnace, inserted from below into the furnace chamberwhich is heated to sintering temperature and located inside the pressurechamber through a charging opening, the pressure chamber being closedand a pressure medium being pumped into the pressure chamber whilesetting the pressure and temperature for sintering, for a predeterminedlength of time, which furnace comprises a cylindrical pressure chamberincluding a high pressure cylinder with a furnace chamber therein, upperand lower end closures projecting into said cylinder, insulation in thepressure chamber between said furnace chamber and the walls of thepressure chamber, which insulation comprises an insulating sheath withinsulating lid and bottom, and in which the upper end closure of thepressure chamber is permanently secured in the high pressure cylinder,the insulating sheath and the insulating lid are suspended in the upperend closure and the lower end closure supports the insulating bottom andthe charge and can be raised and lowered while the furnace is beingcharged from below.
 2. Furnace according to claim 1, in which resistanceelements to heat the furnace are suspended in the upper end closure. 3.Furnace according to claim 1 in which the insulating sheath is connectedin a gas-tight manner to the upper end closure.
 4. Furnace according toclaim 3, in which there is a space between the insulating lid and theupper end closure and the furnace chamber through a pressure-equalizingopening connecting said space to the furnace chamber.
 5. Furnaceaccording to claim 1, comprising a tube suspended in the insulating lidand surrounding the furnace and resistance elements to heat the furnacesupported by said tube.
 6. Furnace according to claim 5, in which theresistance elements are arranged on the outside of the supporting tube.7. Furnace according to claim 6, comprising vertical rails withhorizontal slots facing the tube, the resistance elements beingsupported in said rails and passing through said slots.
 8. Furnaceaccording to claim 7, in which the vertical rails are carried by thetube carrying the resistance elements.
 9. Furnace according to claim 6,in which the tube carrying the resistance elements is suspended in theinsulating lid by means of said rails and at a distance from the lid sothat openings are formed between lid and tube between the rails. 10.Furnace according to claim 5, in which there are gas-tight bushings inthe insulating lid for the resistance elements.
 11. Furnace according toclaim 1, including supply conduits for said resistance elements passingthrough the upper end closure.
 12. Furnace according to claim 1, inwhich the lower end closure comprises a ring-shaped part permanentlysuspended in the high pressure cylinder and a lid inside thisring-shaped part which can be raised and lowered and which supports theinsulating bottom and the charge.
 13. Furnace according to claim 12, inwhich the ring-shaped part of the end closure supports an insulatingring which projects into the insulating sheath so that between this andthe ring a narrow gap is formed.
 14. Furnace according to claim 12, inwhich in the ring-shaped part of the end closure is arranged aring-shaped axially lowerable radiation protection which, when the lidis lowered, is lowered and protects the sealing surface in thering-shaped part from radiation from the charge.